1. Field of the Invention
The present invention relates to an excitation unit for a fiber laser.
2. Description of Related Art
Fiber lasers are a special form of solid state laser. Put simply, a fiber laser, more precisely the resonator of a fiber laser, consists of a glass fiber which has a doped core and a cladding. The doped core of the glass fiber thereby forms the active medium. It is thus a solid state laser with optical waveguide properties. Due to the relatively great length (high aspect ratio) of the fiber laser, the pump light, which is fed into the resonator of the fiber laser through a pump optic, causes an excitation of the active medium.
Fiber lasers are optically pumped in that rays, for example from diode lasers, are coupled in, usually coaxially to the fiber core, into its cladding, or into the core itself. Double clad fibers permit a higher power to be achieved; from the innermost of the two claddings, the pump beam reaches the active fiber core. In most cases, the power of a plurality of diode lasers is combined during the pumping process. Within the technical field, such a device has come to be referred to as a “combiner”. A combiner is a pump module in which several fibers, through each of which the light of a diode laser passes, are spliced onto an excitation fiber.
A further possibility for realizing a pump module consists of an arrangement in which a plurality of laser diodes is so arranged that their output beams are parallel to one another and lie in the same plane. This adjacent arrangement of laser diodes creates a so-called laser bar. If several laser bars are stacked on top of one another, this creates a laser stack.
A fiber laser essentially consists, for example, of one or more pump laser diodes, an input coupling optic (fiber-coupled diode laser, separate or spliced to the cladding) and a resonator. The term “excitation unit”, as used in this description, comprises one or more pump light sources (for example laser diodes in the form of a combiner or the aforementioned laser bars) and the optical and mechanical components which are necessary in order to couple the pump light into the active fiber.
The resonator can be structured in different ways: either it consists of two additional mirrors, which can for example be the two mirrored fiber end surfaces; an arrangement of lens and normal laser mirrors in the reflector and/or output coupling region is also possible. Often, Bragg gratings (also: FBGs=fiber Bragg gratings) are also provided which are inscribed in the waveguide by means of ultraviolet radiation (for example by a 248 nm excimer laser). This leads to lateral differences in the refractive index within the fiber core, with regions with high and low refractive indices which reflect rays of a particular wavelength depending on the period length. The advantage of this is that no additional coupling losses occur on these gratings in a monomode or single-mode fiber and the Bragg gratings only reflect the desired wavelengths selectively. This makes possible an extremely narrow-band operation of the laser. Multimode fibers >20 μm lead to more modes which contain a wider bandwidth of rays. The wavelength range can be reduced by means of external mirrors.
After exiting the active fiber, the laser beam passes into a transport fiber or into a fiber optic cable containing such a fiber. This process takes place with high efficiency, because there are virtually no coupling losses. The rays are, for example, passed via the fiber optic cable to the focusing optics of a laser material processing machine.
A fiber laser also contains the power supply and cooling for the pump laser diodes, as well as other heat-dissipating devices.
Sometimes, high performance fiber lasers possess, in addition, a small fiber laser or a powerful laser diode, which are referred to as seed lasers and which serve to generate the input power for a downstream fiber amplifier (optically pumped active fiber). The division of the laser into seed laser and post-amplification has the advantage that the laser activity can be better controlled. This applies to the wavelength stability, the beam quality and the power stability or pulseability. Usually there is an optical insulator between the seed laser and amplifier fiber.
Depending on the diameter of the fiber core, the laser beam emitted from the core (for example na=0.06) has, for example, a total angle of approximately 5°-10° (na—0.05-0.1). The beam quality is high; the mode of the beam is generally a TEM00 mode, a so-called monomode or single mode, preferred by laser manufacturers and in industry due to its good properties for welding, cutting, drilling, etc.
DE 26 46 692 A1 shows a liquid laser with excitation light sources 5, 6 in the form of flash lamps which are each housed in an elliptical chamber. Between the two elliptical chambers there are two focusing lenses 15, 16 which hold an optical resonator 4 between them. The resonator 4 is pumped by means of the excitation light sources, i.e., the pump light runs from the excitation light sources located in the excitation chambers to the resonator.
DE 198 33 166 A1 describes a pump light input arrangement for laser active fibers. Such a laser active fiber is illustrated in FIGS. 1a and 1b of this document: it is accommodated spirally within a tubular volume, laser diodes 13 project the pump light perpendicularly onto the fiber, which is in the form of a complete fiber with sheathing. The efficiency of the light in-coupling is likely to be extremely poor in this case due to the presence of the sheathing of the laser active fiber. There is no focusing lens between the laser diodes and the fiber.
DE 39 43 722 C2 shows a conventional in-coupling of a pump light into a laser medium 2. By way of example, examples are shown in FIGS. 1a to 1c in which a pump light 42 is directed from a laser diode 41 onto the surface of the laser medium 2. The laser diodes are oriented perpendicularly to the laser medium, the light guide devices always extending in such a way that the pump light enters the laser medium at an acute angle to this. The point at which in-coupling takes place is frequently referred to as the splice point.